Temperature and light adaptations of Synechococcus isolates from a hot spring microbial community

Abstract

Previous molecular analysis of a well-studied microbial mat system in Yellowstone National Park revealed numerous genetically distinct 16S rRNA sequences distantly related to the 16S rRNA sequence of the unicellular cyanobacterium Synechococcus lividus. These new genotypes were shown to be contributed by the predominant cyanobacterial populations. Patterns in genotype distribution relative to temperature and light conditions suggested that these populations may have evolved through adaptive radiation to fill ecological niches. In order to test this hypothesis, Synechococcus isolates were cultivated using a dilution and filtration approach, then shown to be genetically relevant compared to natural mat populations by similarities of 16S rRNA genes and the 16S-23S intervening transcribed spacer (ITS) regions. Several isolates were retrieved that were identical or closely related to predominant mat genotypes at both loci. Other Synechococcus isolates were relevant at the 16S rRNA locus, but had ITS sequences not yet found in the mat.

The growth rate and photosynthetic response of one representative of each genotype was then measured under various temperature and light conditions. Isolates with predominant mat genotypes had distinct temperature ranges and optima for growth, suggesting that specific temperature adaptations exist for these organisms. Isolates with non-predominant genotypes exhibited different temperature ranges and optima that may not be representative of relevant mat populations. Isolates with non-predominant genotypes also grew more slowly, which may signify a lower fitness in situ. Temperature effects on 14CO2 fixation did not exactly reflect temperature relations for growth, but they were consistent with a higher upper temperature limit of the most thermally stable genotype. Growth rate and photosynthetic responses of the isolates to light did not provide conclusive evidence of light adaptation; however, there was evidence for acclimation to light.